For example, a hybrid vehicle loaded with a traction motor has an inverter connected to the motor, a main battery connected to the inverter, a system main relay (SMR) electrically connecting or disconnecting the inverter and the main battery, and a capacitor connected between direct current (DC) voltage terminals of the inverter. Generally, the above hybrid vehicle has a parallel circuit of a pre-charge relay, to which a pre-charge resistor for limiting inrush current is connected in series, and an SMR, to which the pre-charge resistor is not connected. In an initiation of the vehicle, the SMR is turned on after the pre-charge relay is turned on to pre-charge the capacitor. As such, the SMR is restricted from being damaged by large current.
Similarly to a vehicle only having an engine as a driving source, the hybrid vehicle is loaded with an auxiliary machine that receives electric power from an auxiliary battery and that outputs voltage lower than the main battery. In such a hybrid vehicle loaded with the auxiliary battery, a bidirectional DC to DC (DC-DC) converter or a DC-DC converter converting low voltage into high voltage is disposed between the main battery and the auxiliary battery. When such a DC-DC converter is disposed and the capacitor is pre-charged by the DC-DC converter, the pre-charge relay may be omitted. In this case, when the target voltage of the pre-charge is set to be equal to the voltage of the main battery and the SMR is turned on after the pre-charge is finished, the SMR is restricted from being damaged. However, depending on a detection error of a voltage sensor, which detects voltage, and depending on accuracy of output voltage of the DC-DC converter, there is a possibility that the voltage of the capacitor after the pre-charge is different from the voltage of the main battery and large current flows when the SMR is turned on. In such a case, there is difference between voltage of the capacitor right before the SMR is turned on and voltage of the capacitor right after the SMR is turned on.
A control device for a power supply system disclosed in JP 2007-209114 A detects a prior voltage and a subsequent voltage of the capacitor, the prior voltage being a voltage of the capacitor after the pre-charge is finished and right before the SMR is tuned on, and the subsequent voltage being a voltage of the capacitor right after the SMR is turned on. The control device records difference between the prior voltage and the subsequent voltage of the capacitor as a correction value. The control device corrects a target voltage for a next pre-charge using the recorded correction value and controls the difference between the voltage of the main battery and the voltage of the capacitor after the pre-charge. Furthermore, the control device restricts large current from flowing in the SMR.